
\(\eta \) , the conjugate momentum to the number coordinate, admits a
variety of interpretations. In the construction of the equilibrium
free energy of a chemical mixture, it has the dimensions of a chemical
potential [1]}, so we will refer to \(\left( \eta ,n \right)\)  as number-potential coordinates. Both \(\xi \)  and \(n\)  have
dimensions of species numbers, and one of the subtleties of Doi-Peliti
theory is understanding the difference of their meaning in WKB-type
approximations. The eikonal approximation for moment ratios is
interesting because it establishes a relation between the two in the
same observable.
